Many glass bottles have conventionally been used as containers for drinks such as soft drinks and alcohols. Metal caps called crown caps are widely used for narrow-mouthed glass bottles. In general, a crown cap is manufactured from a steel sheet by press forming and includes a disk-shaped portion for covering the mouth of a bottle and a pleated portion placed therearound. The bottle is tightly sealed by crimping the pleated portion to the mouth of the bottle.
Contents, such as beer and carbonated drinks, causing an internal pressure are often filled in bottles for which crown caps are used. Therefore, the crown caps need to have high pressure resistance such that the seal of the bottles is not broken by the deformation of the crown caps when the internal pressure is increased by a change in temperature or the like. Furthermore, even if the strength of the material is sufficient, when the material has poor formability, the shape of pleats becomes non-uniform; hence, even if a pleated portion is crimped to the mouth of a bottle, sufficient airtightness can not be obtained in some cases. Therefore, the crown caps need to have excellent formability.
A steel sheet used to manufacture crown caps is mainly an SR (single-reduced) steel sheet. This is obtained in such a manner that a steel plate is thinned by cold rolling, is annealed, and is then temper rolled. The thickness of a steel sheet for conventional crown caps is generally 0.22 mm or more and sufficient pressure resistance and formability have been capable of being ensured by the use of an SR material made of mild steel used to for cans for foods and drinks.
In recent years, a reduction in the thickness has been increasingly required for steel sheets for crown caps, as well as steel sheets for cans, for the purpose of cost reduction. When the thickness of a steel sheet for crown caps is 0.20 mm or less, a crown cap manufactured from a conventional SR material is short of pressure resistance. In order to ensure the pressure resistance, it is conceivable to use a DR (double-reduced) steel sheet which is obtained by performing secondary cold rolling after annealing and which can take advantage of work hardening compensating for a reduction in strength due to the reduction of the thickness. An increase in rolling reduction during secondary cold rolling hardens a steel sheet to reduce the formability thereof. In the formation of a crown cap, a central portion is drawn to a certain degree early in the formation thereof and an outside edge portion is then formed into a pleated shape. In the case of a steel sheet with low formability, a shape failure in which the pleated shape is non-uniform occurs in some cases. A crown cap with a non-uniform pleated shape has a problem that pressure resistance can not be obtained by capping a bottle, contents leak, and the crown cap does not play a role as a lid. When the strength of a steel sheet is low, a crown cap may possibly be detached due to insufficient pressure resistance even if the pleated shape thereof is uniform.
In order to obtain a steel sheet having both excellent strength and formability in the reduction of thickness, techniques below have been proposed.
Patent Literature 1 discloses a soft steel sheet, excellent in can strength and can formability, for containers. The soft steel sheet contains N: 0.0040% to 0.0300% and Al: 0.005% to 0.080% on a mass basis and has a 0.2% yield strength of 430 MPa or less as determined by a tensile test using a JIS No. 5 test specimen, a total elongation of 15% to 40%, a Q−1 of 0.0010 or more due to internal friction, and a thickness of 0.4 mm or less.
Patent Literature 2 discloses a high-strength, high-workability steel sheet for cans. The steel sheet contains C: 0.001% to 0.080%, Si: 0.003% to 0.100%, Mn: 0.10% to 0.80%, P: 0.001% to 0.100%, S: 0.001% to 0.020%, Al: 0.005% to 0.100%, N: 0.0050% to 0.0150%, and B: 0.0002% to 0.0050% on a mass basis and also contains crystal grains having an elongation rate of 5.0 or more in a rolling-direction cross section at an area fraction of 0.01% to 1.00%.